This invention relates generally to apparatus and methods for thermally accelerating temperature related failure mechanisms in semiconductor electronic devices or components, such as discrete transistors and integrated circuits, and particularly to such apparatus and methods in which the semiconductor devices are carried on circuit boards and are immersed in a liquid, heat exchanging medium for obtaining desired thermal conditions.
Semiconductor devices, such as discrete transistors and integrated circuits have time and temperature dependant failure mechanisms that occur at a high percentage rate during the first year of regular service. Thereafter, the surviving devices exhibit a low and relatively constant failure rate over the remainder of their useful lives of many years.
This high percentage failure rate is referred to as "infant mortality", since it occurs mainly during the first year of the life of a semiconductor device. Those devices that will fail during the first year can be substantially eliminated from commercially available products by subjecting the devices to precise elevated thermal conditions under electrical operating conditions. This procedure, or testing, called "burn-in", effectively accelerates the devices through one year of normal operation in approximately 168 hours, or one week.
Recently the burn-in procedure has been effected in a liquid, heat exchange or transfer medium. The semiconductor devices are loaded onto circuit cards and the cards are immersed in a bath of the liquid medium elevated to the desired temperature. The devices then are electrically operated for a period determined by a mathematical equation relating the operating parameters, including temperature, to accelerated life of the devices. The heat exchange medium, thus, serves to increase the ambient operating temperature of the devices and to dissipate received excess operating heat produced by the operating devices to maintain the devices at a precise, elevated ambient temperature.
The heat exchange medium typically is that known under the trademark Fluorinert liquid FC-40 manufactured by 3M Commercial Chemicals Division and is well known to be non-reactive and electrically insulating in such an environment. This medium has a specific gravity greater than water, a specific heat greater than air and is selected to have a boiling point greater than the elevated temperature at which the burn-in procedure is effected. Other silicone based liquid mediums also can be used.
Use of a liquid medium improves upon the use of gas mediums, such as air and nitrogen, in burn-in procedures and apparatus in that, among other things, the selected liquid medium has a greater specific heat or increased capacity to hold heat produced by the devices, can provide improved temperature control of the bath environment, especially with a high density of energy dissipation, and can absorb increased quantities of heat when vaporized by a localized "hot spot", such as a failed device, to prevent thermal runaway of that failed device This last feature provides for continuance of the burn-in procedure without having to stop for removal of a single, failed device. A Fluorinert medium has some qualities, however, that must be specially cared for.
First, Fluorinert medium, which is unusually expensive, has a low evaporation rate at room temperature, but at the elevated temperatures of a burn-in procedure, has an evaporation rate that is significant. A burn-in effecting apparatus must thus provide for recovery of the evaporated liquid medium vapors to be cost efficient. The recovery must be performed carefully, however, because the Fluorinert medium vapors readily combine chemically with any included moisture to form hydrofluoric acid, which can erode the stainless steel and plastic components of the burn-in effecting unit Thus, condensed water vapor must be separated from the condensed Fluorinert heat exchange medium vapor prior to return of the Fluorinert medium liquid to the bath.
Second, the Fluorinert medium must be controlled in temperature by a heater to raise the medium to the desired elevated temperature for effecting the burn-in procedure and by a cooler, such as a mechanical refrigeration unit, to remove excess heat produced by the operating devices from the bath medium during the burn-in procedure to maintain the elevated temperature and possibly to cool the medium at the end of the procedure.
Third, filtration must be provided to remove solid particulate and chemical contaminants, such as solder flux, debris and acids, introduced into or produced in the bath medium. Since the Fluorinert medium is twice the density of water, many solid contaminants can float on top of the medium in addition to other solid contaminants being suspended in the medium. Previously, the filtration systems effected a flow of the medium through the filter element and only removed the suspended solid particles. The floating particles never passed through the filtration system because they never sank in the medium to the filtration port. The floating contaminants instead were removed manually by an operator scooping them off the top surface of the medium with such as filter paper. Although inconvenient, this had been satisfactory since the semiconductor devices were continuously submerged and were surrounded by filtered medium and the floating contaminants were removed manually before removing the devices from the bath. Further, the devices were sealed so that minute solid, particulate contaminants had no effect on the operation of the devices.
A problem has recently arisen, however. Some users of the burn-in bath apparatus want to effect the burn-in procedures on semiconductor devices that are in an intermediate stage of production, that is to say, the covers have not yet been applied to the lead frames and the bare circuits are directly exposed to the bath medium. This can be advantageous to a manufacturer because, in complex and expensive hybrid devices carrying several different chips, a burn-in failure at such an intermediate stage of manufacture can be repaired and a major portion of the device can be salvaged. A burn-in failure after the covers are sealed to the lead frames requires that the entire device be discarded, including the good portions. This old practice is of course wasteful and costly.
Conducting the burn-in procedure on un-covered devices presents little problem regarding contaminants suspended in the medium because suspended solid particles are well filtered from the medium and the uncovered devices remain clean, which is critical to their operation. The problem is removing the uncovered devices from the bath medium through the contaminants floating on top of the dense bath medium. A solution to this problem is necessary to be able to burn-in devices in an intermediate stage of production.
Lastly, such a burn-in apparatus should have a bath tank large enough to facilitate burn-in procedures of production quantities of semiconductor devices and be self contained upon connection to an electrical power supply. The apparatus should also provide easy circuit card handling.